1. Field of the Invention
This invention relates to optical apparatus (e.g., devices, systems) that trap light pulses at predetermined locations, and, more particularly, to optical memories, buffers and switches that trap gap solitons at controlled perturbations in periodic optical structures.
2. Discussion of the Related Art
Optical solitons are important carriers of optical energy in many physical systems. The emergence of solitons is understood to be a consequence of the balance of dispersive and nonlinear effects on the same length scale. More specifically, it is well known that dispersion causes optical (light) pulses to spread out as they propagate in a dispersive medium (e.g., a silica fiber) due to the fact that the refractive index of the medium is wavelength dependent; thus, different wavelength components of the pulse travel at different speeds. On the other hand, the electric-field-dependent nonlinearity of the refractive index of the same medium may compensate for this pulse spreading at sufficiently high intensity of the propagating pulse. When this compensation or balancing is controlled properly, the pulse will retain its original shape, without spreading, over extended propagation distances. This type of pulse is referred to as a temporal soliton.
Optical temporal solitons have been considered candidates for the data bits by which to information is transferred over long distances. In contrast, recent advances in the fabrication of optical fiber with microstructure have rendered the possibility of storing information in the form of optical gap solitons, which are nonlinear bound states that propagate in periodic structures. Their frequencies lie in the band gap of the linear (Floquet-Bloch) frequency spectrum. They have been anticipated in theoretical work [See, for example, W. Chen et al., Phys. Rev. Lett., Vol. 58, p. 160 (1987), which is incorporated herein by reference.] and observed in experimental work [See, for example, B. J. Eggleton et al., J. Opt. Soc. Am. B. Vol. 14, No. 11, p. 2980 (1997), which is also incorporated herein by reference.] on sufficiently high intensity light propagation in optical fiber with a periodically varying refractive index (specifically, a uniform fiber grating). In contrast to optical fiber used in long distance communication systems, where the formation length for temporal solitons is on the order of kilometers, the formation length for gap solitons is on the order of only centimeters.
We have discovered that a gap soliton can be controlled by introducing a perturbation in an optical structure (e.g., a waveguide) that includes an otherwise essentially periodic variation of its refractive index. In one embodiment of our invention, by controlling the amplitude, phase and/or average (e.g., background) value of the refractive index of a first perturbation, and by maintaining the speed of the soliton below a certain critical speed, we have found that the soliton will transfer its energy to the modes of the perturbation. When the soliton transfers essentially all of its energy in this way, we refer to the soliton as being trapped or captured by the perturbation. Surprisingly, the soliton can be trapped without loss of a significant amount of its energy. In an alternative embodiment, when the speed of the soliton exceeds the critical speed, a second perturbation is introduced into the waveguide structure to reduce the speed of the soliton, thereby enabling its capture by the first perturbation. In another embodiment, a gap soliton is captured between two perturbation regions neither of which is capable of capturing the soliton by itself.
Our approach to trapping gap solitons enables a variety of unique applications in, for example, memories, buffers, switches and WDM demultiplexers.